The present invention relates to circuits wherein input power is interrupted and, more particularly, to a circuit of the type described which sustains output power during said input power interruption.
A circuit for the purposes described is disclosed in U.S. Pat. No. 4,593,213 which issued to Vesce, et al on Jun. 3, 1980. Vesce discloses an electronic device comprising a switched mode power supply and a hold-up circuit. The object of the Vesce device is to transfer high voltage energy stored in a low capacitance bank to a low voltage bus, with minimum electrical disturbance, using a current-limited MOSFET switch. The power supply includes a hold-up series diode and a pulse width modulated power switch. The power supply receives primary input power. The hold-up circuit includes a high voltage energy storage capacitor bank and a hold-up transfer switch. The switch transfers high voltage energy stored in the storage capacitor bank to the low voltage bus of the power supply upon receiving an enable signal from a hold-up enable circuit. The hold-up transfer switch includes a zener diode, a resistor, a P-channel power MOSFET switch, and a diode. The Vesce device wastes energy in the disclosed switch because the switch is not operated in its saturation region. The Vesce device also requires an extra diode which further increases the losses in the circuit.